Power oscillator apparatus with transformer-based power combining

ABSTRACT

An oscillator circuit includes first and second oscillators arranged in a series configuration between a supply voltage node and a reference voltage node. The first and second oscillators are configured to receive a synchronizing signal for controlling synchronization in frequency and phase. An electromagnetic network provided to couple the first and the second oscillators includes a transformer with a primary circuit and a secondary circuit. The primary circuit includes a first portion coupled to the first oscillator and second portion coupled to the second oscillator. The first and second portions are connected by a circuit element for reuse of current between the first and second oscillators. The oscillator circuit is fabricated as an integrated circuit device wherein the electromagnetic network is formed in metallization layers of the device. The secondary circuit generates an output power combining power provided from the first and second portions of the primary circuit.

PRIORITY CLAIM

This application is a divisional application from U.S. application forpatent Ser. No. 14/216,037 filed Mar. 17, 2014, which claims priorityfrom Italian Application for Patent No. MI2013A000454 filed Mar. 26,2013, the disclosures of which are incorporated by reference.

TECHNICAL FIELD

The present disclosure relates to a power oscillator apparatus withtransformer-based power combining

BACKGROUND

It is known in the state of the art the use of circuit apparatuscomprising at least two oscillators coupled by means of a propernetwork. The main applications of such an apparatus are theimplementation of both quadrature signals and voltage-controlledoscillators with low phase-noise. For this approach, the design of thecoupling network is the main issue. The coupling network may be of theactive type, as disclosed in Jeong Ki Kim et al., “A current-reusequadrature VCO for wireless body area networks,” IEEE/NIH LiSSA, pp.55-58, 2011 (the disclosure of which is incorporated by reference), orcapacitive type as disclosed in Oliveira, L. B. et al., “Synchronizationof two LC- oscillators using capacitive coupling,” IEEE ISCAS, pp.2322-2325, 2008 (the disclosure of which is incorporated by reference),or inductive type as disclosed in Tzuen-Hsi Huang et al., “A 1 V 2.2 mW7 GHz CMOS quadrature VCO using current-reuse and cross-coupledtransformer-feedback technology,” IEEE MWCL, vol. 18, pp. 698-700,October 2008 (the disclosure of which is incorporated by reference).

Also it is known in the state of the art the use of power combiningtechniques to increase the overall output power in several applications.Due to technology limits, (e.g., breakdown, electro-migrationconstraints, thermal issues, etc.) the power level delivered by a singlepower stage is often below the application requirements, thus amultistage solution is required. When it comes about dc/ac conversion,transformer-based power-combining is the straight-forward solution. Anexample of power-combining system is disclosed in Tomita et al., “1-W3.3-16.3-V boosting wireless power oscillator circuits with vectorsumming power controller,” IEEE JSSC, vol. 47, pp. 2576-2585, November2012 (the disclosure of which is incorporated by reference), where twopower stages separately drive two series resonant circuits and bothdrivers are magnetically coupled with the secondary inductance. Bycontrolling the phase relation between the driver's signals, the outputpower can effectively reach two times the power delivered by a singlestage.

SUMMARY

One aspect of the present disclosure is to provide a power oscillatorapparatus with transformer-based power combining which is able todeliver higher levels of output power with high efficiency compared toknown prior art apparatus.

One aspect of the present disclosure is a power oscillator apparatuscomprising: a first power oscillator and a second power oscillatorarranged in series between a supply voltage and a reference voltage, anelectromagnetic network for coupling the first and the secondoscillator, characterized by comprising a transformer with a primarycircuit including a first portion connected to the first oscillator andsecond portion connected to the second oscillator, a circuit element forreusing the current used in the first oscillator even into the secondoscillator, an output stage of the apparatus comprising a secondarycircuit of the transformer, the first and the second oscillator beingconfigured to receive a synchronizing signal for their synchronizationin frequency and phase and said secondary circuit being magneticallycoupled with the first and the second portion of the primary circuit toobtain an output power as combination of a first power associated to thefirst portion and a second power associated to the second portion of theprimary circuit.

In an embodiment, an apparatus comprises: a first oscillator circuithaving a first output and a second output; a second oscillator circuithaving a third output and a fourth output; and a transformer circuitcomprising: a first primary winding coupled between the first output anda common node; a second primary winding coupled between the secondoutput and the common node; a third primary winding coupled between thethird output and the common node; a fourth primary winding coupledbetween the fourth output and the common node; and a first secondarywinding and second secondary winding coupled in series between fifth andsixth output nodes, wherein the first secondary winding is magneticallycoupled to the first and third primary windings, and wherein the secondsecondary winding is magnetically coupled to the second and fourthprimary windings.

In an embodiment, an apparatus comprises: a first oscillator circuithaving a first output and a second output; a second oscillator circuithaving a third output and a fourth output; a transformer having aprimary winding including a first portion coupled between the first andsecond outputs of the first oscillator circuit and a second portioncoupled between the third and fourth outputs of the second oscillatorcircuit, and further including a secondary winding having a thirdportion coupled in series with a fourth portion, wherein the thirdportion is magnetically coupled to the first and second portions of theprimary winding and wherein the fourth portion is magnetically coupledto the first and second portions of the primary winding; wherein thefirst and second oscillator circuits and the transformer are integratedin an integrated circuit device including a plurality of metallizationlevels; and wherein said primary and secondary windings are formed insaid plurality of metallization layers.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of the present invention, a preferredembodiments thereof are now described, purely by way of non-limitingexample and with reference to the annexed drawings, wherein:

FIG. 1 shows a power oscillator apparatus according to the presentdisclosure;

FIG. 2 shows a power oscillator apparatus according to a firstembodiment of the present disclosure;

FIG. 3 shows a a power oscillator apparatus according to a secondembodiment of the present disclosure;

FIG. 4 shows a power oscillator apparatus according to a thirdembodiment of the present disclosure;

FIG. 5 shows the time diagrams of the some voltages of the poweroscillator apparatus in FIG. 2;

FIG. 6 shows more in detail the synchronizing circuit in FIGS. 2-4;

FIG. 7 is a schematic tridimensional view of an implementation of thestructure of the transformer of FIG. 2;

FIG. 8 is schematic planar view of the implementation of the structureof the transformer of FIG. 7;

FIG. 9 is schematic planar view of another implementation of thestructure of the transformer of FIGS. 2 and 3;

FIG. 10 is schematic planar view of an implementation of the structureof the transformer of FIG. 4; and

FIG. 11 is schematic planar view of another implementation of thestructure of the transformer of FIG. 4.

DETAILED DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a power oscillator apparatus according to the presentdisclosure.

The power oscillator apparatus comprises a first power oscillator POSCand a second power oscillator NOSC arranged in series between a supplyvoltage VDD and a reference voltage, for example ground GND.

The power oscillator apparatus comprises an electromagnetic network 100configured to couple the oscillators POSC and NOSC each one having twooutput terminals OUT1, OUT2.

The power oscillator apparatus comprises a transformer 50. The primarycircuit 51 comprises a first portion 52 connected to the firstoscillator POSC and a second portion 53 connected to the secondoscillator NOSC; the first portion 52 is connected with the outputterminals OUT1, OUT2 of the first oscillator POSC while the secondportion 53 is connected with the output terminals OUT1, OUT2 of thesecond oscillator NOSC.

The primary circuit of the transformer comprises preferably four primarywinding inductors L_(P1)-L_(P4) wherein the first portion 52 comprisestwo winding inductors and the second portion 53 comprises the other twowinding inductors.

The power oscillator apparatus comprises a circuit element 101 to allowthe reuse of the current I passing through the first oscillator eveninto the second oscillator NOSC; the circuit element 101 is preferablythe common center tap of the first 52 and second 53 portion of theprimary circuit 51 of the transformer 50.

The power oscillator apparatus receives a synchronizing signal Ipulsefor the synchronization in frequency and phase of the first POSC and thesecond NOSC oscillators; the synchronizing signal Ipulse derives from asynchronizing circuit 60, preferably included in the power oscillatorapparatus. The synchronization frequency f_(sync) of the thesynchronizing signal Ipulse is equal about to 2*f_(osc) is theoscillation frequency of the each oscillator NOSC, POSC. Thesynchronizing circuitry 60 forces the oscillators POSC and NOSC tooperate in phase, so that the voltages applied across the winding orcoils L_(P1)-L_(P4), denoted with the same symbol (i.e., dot or cross),are at the same time all positive or all negative.

The power oscillator apparatus comprises an output stage 70 includingthe secondary circuit L_(S1) and L_(S2) of the transformer which isconnectable with a load LOAD, for example a rectifier. The secondarycircuit L_(S1), L_(S2) is magnetically coupled with the primary circuitto obtain an output power Pout which is a power combining of a firstpower P1 associated to the first portion 52 of the primary circuit and asecond power P2 associated to the second portion 53 of the primarycircuit. The transformer 50 allows the galvanic isolation between theoscillators NOSC, POSC and the output stage 70 of the power oscillatorapparatus.

FIG. 2 shows a power oscillator apparatus according to a firstembodiment of the present disclosure. The oscillators POSC and NOSC areimplemented by complementary oscillators; the oscillators POSC and NOSCmay be implemented in either bipolar or CMOS technologies. FIG. 2 showsthe oscillators POSC and NOSC implemented in CMOS technologies.

The oscillator POSC comprises a first PMOS transistor M1 and a secondPMOS transistor M2 which have the source terminals connected to thesupply voltage VDD and are cross-coupled, that is the gate terminal ofthe transistor M1 is in common with the drain terminal of the transistorM2 and the gate terminal of the transistor M2 is in common with thedrain terminal of the transistor M1.

The oscillator NOSC comprises a first NMOS transistor M3 and a secondNMOS transistor M4 which have the source terminals connected to groundGND and the gate terminals connected by means of the resistances R3 andR4 with the bias voltage V_(B) at the bias terminal Pbias. Theoscillator NOSC comprises a capacitor C3 connected with the gateterminal of the transistor M3 and the drain terminal of the transistorM4 and another capacitor C4 connected with the gate terminal of thetransistor M4 and the drain terminal of the transistor M3.

The electromagnetic network 100 configured to couple the oscillatorsPOSC and NOSC is of the inductive type and comprises the primary circuit51 of the transformer 50. The primary circuit 51 comprises the firstportion 52 including the series of coils L_(P4) and L_(P3) associated tothe oscillator POSC and the second portion 53 including the series ofthe coils L_(P1) and L_(P2) associated to the oscillator NOSC; thecoupling between the oscillators POSC and NOSC is assured by themagnetic coupling of the coils L_(P4) and L_(P2) denoted by the symbolcross and the magnetic coupling of the coils L_(P1) and L_(P3) denotedby the symbol dot.

A capacitor C1 is connected between the drain terminals of thetransistors M1 and M2 and forms with the coils L_(P4) and L_(P3) aresonant tank LC while a capacitor C2 is connected between the drainterminals of the transistors M3 and M4 and forms with the coils L_(P1)and L_(P2) another resonant tank LC.

The secondary circuit of the transformer 50 comprises the series of thecoils L_(S1) and L_(S2) wherein the coil L_(S1) is magnetically coupledwith the coils L_(P1) and L_(P3) of the primary circuit and the coilL_(S2) is magnetically coupled with the coils L_(P2) and L_(P4) of theprimary circuit. The output power Pout relative to the series of thecoils L_(S1) and L_(S2) is a power combining of each power contributionP_(LP1)-P_(LP4) of the respective coil L_(P1), L_(P2), L_(P3) and L_(P4)of the primary circuit 51.

When the transistor M1 is on and the transistor M2 is off the current Iflows through the coils L_(P4) and L_(P2) and the transistor M4 whilewhen the transistor M2 is on and the transistor M1 is off the current Iflows through the coils L_(P3) and L_(P1) and the transistor M3. Thevalues of inductors L_(P2), L_(P3) L_(P4) and capacitors C 1 and C2 arerelated to the oscillation frequency f_(OSC) that is typically in therange between hundreds of megaHertz to several gigahertz. Therefore, ina typical integrated implementation of the proposed solution inductorsand capacitors of a few nanoHerny and picoFarad are used, respectively.

The synchronizing circuit 60 uses common-mode current pulses Ipulse. Thecurrent pulses are injected into the power oscillator apparatus by usinga common-mode bias terminal Pbias, which can be placed in either theoscillators NOSC or POSC and which, in FIGS. 2-4, is arranged in theoscillator NOSC; preferably a NMOS transistor M6 coupled between thecircuit 60 (coupled with the supply voltage VDD) and ground GND and withthe gate and drain terminal in common and with the drain terminalconnected with the circuit 60, allows the use of the terminal Pbias forthe injection of the current pulses Ipulse. Current pulses Ipulse have afrequency f_(sync) approximately equal to two times the oscillationfrequency f_(osc) of the oscillator POSC, NOSC; current pulses Ipulsehave preferably a square-wave shape. Preferably the synchronizingcircuitry 60, as shown in FIG. 6, includes a low-power low-accuracyvoltage oscillator 61 (e.g., a ring oscillator), a voltage-to-currentconverter 62 receiving the voltage pulses output from the oscillator 61,and a current buffer 62 receiving the current pulses Ipulse, for exampleof 1 mA, from the converter 62 and adapted to inject the current pulsesIpulse into the bias terminal Pbias.

The presence of the synchronizing signal Ipulse of the synchronizingcircuit 60 is mandatory to avoid NOSC and POSC work at differentfrequency/phase, thus hindering the power-combining at the output stage70. The synchronizing circuit 60 drives the second-harmonic(common-mode) current component to both NOSC and POSC, thus settingfrequency/phase of NOSC and POSC.

The synchronization signal has no impact on the oscillator efficiencysince low-value current pulses are required and synchronization is onlyrequired at the circuit start-up. Indeed, after the oscillator is lockedin a stable state, it remains indefinitely in this state, regardlesssignal disturbance.

FIG. 5 shows a the typical waveforms of the voltages at the coils of thepower oscillator apparatus in FIG. 2; Vout—NOSC is the differentialvoltage across the oscillator NOSC, i.e. the voltage across the seriescombination of L_(P1) and L_(P2) and Vout—POSC is the differentialvoltage across the oscillator POSC, i.e. the voltage across the seriescombination of L_(P3) and L_(P4) while Vout is the differential voltageacross the equivalent load LOAD, i.e. the voltage across the seriescombination of the secondary coils L_(S2) and L_(S1) which is greaterthan the voltages Vout—POSC and Vout—NOSC but smaller than their sum.

It is clearly shown that due to the phase-relationship between Vout—NOSCand Vout—POSC, the currents forced at the primary coils are at the sametime all increasing or all decreasing, and hence the fluxes generated atthe primary coils. It follows that the secondary coils will catch thisflux (separately, i.e. L_(S1) will catch the flux generated by L_(P1)and L_(P3) and so on), forcing to the load a current proportional to thefluxes. At the secondary side the output voltage will be greater thanVout—NOSC or Vout—POSC, depending on the load resistance and thecoupling factor between primary and secondary side, always less thanone. The total power at the load LOAD is the sum of the total powerapplied at the primary side, except for the losses in the seriesresistance of the windings.

FIG. 3 shows a power oscillator apparatus according to a secondembodiment of the present disclosure. Differently from the poweroscillator apparatus in FIG. 2, the electromagnetic network 100configured to couple the oscillators POSC and NOSC of the poweroscillator apparatus in FIG. 3 is of the capacitive type; in fact theelectromagnetic network 100 comprises the capacitor C1 connected betweenthe first output terminal OUT1 of the first oscillator and the secondoutput terminal OUT2 of the second oscillator and a second capacitor C2connected between the second output terminal OUT2 of the firstoscillator and the first output terminal OUT1 of the second oscillator,that is the capacitor C1 is connected between the drain terminal of thePMOS transistor M1 and the drain terminal of the NMOS transistor M4 andthe capacitor C2 is connected between the drain terminal of the PMOStransistor M2 and the drain terminal of the NMOS transistor M3.

Differently from the power oscillator apparatus in FIG. 2, the firstportion 52 of the primary circuit 51 of the transformer 50 comprises theseries of the coils L_(P2) and L_(P4) connected between the drainterminals of the PMOS transistors M1 and M2 and the second portion 53 ofthe primary circuit 51 comprises the series of the coils L_(P1) andL_(P3) connected between the drain terminals of the NMOS transistors M3and M4.

The secondary circuit of the transformer 50 comprises the series of thecoils L_(S1) and L_(S2) wherein the coil L_(S1) is magnetically coupledwith the coils L_(P1) and L_(P3) of the primary circuit and the coilL_(S2) is magnetically coupled with the coils L_(P2) and L_(P4) of theprimary circuit. The output power Pout relative to the series of thecoils L_(S1) and L_(S2) is a power combining of each power contributionP_(LP1)-P_(LP4) of the respective coil L_(P1), L_(P2), L_(P3) and L_(P4)of the primary circuit 51.

FIG. 4 shows a power oscillator apparatus according to a thirdembodiment of the present disclosure. Differently from the poweroscillator apparatus in FIG. 2, the electromagnetic network 100configured to couple the oscillators POSC and NOSC of the poweroscillator apparatus in FIG. 3 is of the capacitive and inductive type;in fact the electromagnetic network 100 comprises the capacitor C1connected between the first output terminal OUT1 of the first oscillatorPOSC and the second output terminal OUT2 of the second oscillator NOSCand a second capacitor C2 connected between the second output terminalOUT2 of the first oscillator and the first output terminal OUT1 of thesecond oscillator, that is the capacitor C1 is connected between thedrain terminal of the PMOS transistor M1 and the drain terminal of theNMOS transistor M4 and the capacitor C2 is connected between the drainterminal of the PMOS transistor M2 and the drain terminal of the NMOStransistor M3.

Also the electromagnetic network 100 comprises the primary circuit 51 ofthe transformer 50. The electromagnetic network 100 comprises the seriesof coils L_(P4) and L_(P3) associated to the oscillator POSC, that isconnected to the output terminals OUT1 and OUT2 of the oscillator POSC,and the series of the coils L_(P1) and L_(P2) associated to theoscillator NOSC, that is connected to the output terminals OUT1 and OUT2of the oscillator NOSC,; the coupling between the oscillators POSC andNOSC is assured by the magnetic coupling of the coils L_(P4) and L_(P2)denoted by the symbol cross and the magnetic coupling of the coilsL_(P1) and L_(P3) denoted by the symbol dot.

For all the embodiments in FIGS. 2-4, the transformer topology comprisestwo separated magnetic circuits, whose common fluxes are marked by dots(i.e., L_(P1,3) with L_(S1)) and crosses (i.e., L_(P2,4) with L_(S2)),respectively. Dots and crosses are placed according to the common fluxconventions.

In accordance with the power oscillator apparatus of the presentdisclosure it is possible to perform an integrated circuit comprisingthe power oscillator apparatus as shown in each one of the FIGS. 1-4.The integrated circuit shows a physical monolithic implementation forthe transformer 50 using only three metal layers. FIG. 7 shows aschematic tridimensional view of the structure of the transformer 50while FIGS. 8-11 show schematic planar views of primary L_(P1)-L_(P4)and secondary L_(S1), L_(S2) windings. FIG. 8 is the planar view oftransformer 50 related to the tridimensional view of FIG. 7. A stackedarrangement for the transformer 50 comprises the primary coilsL_(P1)-L_(P4) performed in the mid-level or intermediate metal layer 55and the secondary coils L_(S1),L_(S2) performed in the top metal layer56; preferably the primary coils L_(P1)-L_(P4) and the secondary coilsL_(S1),L_(S2) are provided in the form or metal spirals. The commoncenter tap 101 may be performed in the bottom metal layer 57 or in theintermediate metal layer 55. The integrated circuit is performed in achip of semiconductor material and the transistors M1-M4 and the otherelements of the oscillators POSC and NOSC except the transformer 50 areperformed according to the known technology.

The four inductors L_(P1)-L_(P4) of the primary coils are arranged usingtwo symmetric interleaved configurations, one for each secondary coupledcoils L_(S1), L_(S2), with a common terminal for the center-tap 101.Underpasses are performed in the bottom metal layer 57 and are only usedto contact the inductors terminals and preferably the center-tap 101.Secondary coils L_(S1), L_(S2) are stacked on top of primary coilsL_(P1)-L_(P4) and series-connected to build the secondary winding. Theirinner terminals are contacted by bonding wires. The primary coilsL_(P1), L_(P3) (with the winding L_(P1) in black and the winding L_(P3)in white) are arranged in a interleaved configuration under thesecondary coil L_(S1) and the primary coils L_(P2), L_(P4) (with thewinding L_(P2) in black and the winding L_(P4) in gray) are arranged ina interleaved configuration under the secondary coil L_(S2).

The stacked configuration between primary and secondary windings isinherently suitable to obtain galvanic isolation, provided that suitabledielectric layer between the intermediate metal layer 55 and the topmetal layer 56 is used. For the sake of clarity, FIGS. 7-11 are only anexample of implementation. Indeed, the shape, the number of turns andthe turn ratio between primary and secondary windings may vary.Moreover, if more metal layers are available, multi-layershunt-connected spirals can be exploited to reduce the seriesresistances of the coils. Patterned ground shields can be implementedbelow the primary windings to reduce substrate losses if necessary.

For both schematics in FIGS. 2 and 3, an alternative implementation ofthe transformer is reported in the planar view in FIG. 9. It mainlydiffers from the one shown in FIG. 8 for the magnetic fields B that arein opposite phase between coils at the left side, L_(P1), L_(P3) andL_(S1), and the right side, L_(P2), L_(P4) and L_(S2), of the structure.This configuration allows lower electromagnetic interferences to beachieved.

For the schematic in FIG. 4 two alternative implementations of thetransformer 50 are shown in FIGS. 10 and 11. These implementations usetwo different interleaved transformers at the primary side, while thesecondary is the same as the previous solutions in FIGS. 8 and 9,respectively. FIG. 10 shows the primary coils L_(P1), L_(P3) arranged inan interleaved configuration under the secondary coil L_(S1) and theprimary coils L_(P2), L_(P4) arranged in an interleaved configurationunder the secondary coil L_(S2) with the magnetic fields B that are inphase between the coils while FIG. 11 shows the primary coils L_(P1),L_(P3) arranged in an interleaved configuration under the secondary coilL_(S1) and the primary coils L_(P2), L_(P4) (with the winding L_(P2) inwhite and the winding L_(P4) in black) arranged in an interleavedconfiguration under the secondary coil L_(S2) with the magnetic fields Bthat are in opposite phase between coils at the left side, L_(P1),L_(P3) and L_(S1), and the right side, L_(P2), L_(P4) and L_(S2), of thestructure.

Compared to the implementations in FIGS. 8 and 9 this arrangement needsonly one underpass.

Compared to the typical apparatuses, the power oscillator apparatusshown in FIG. 1-11 is able to deliver higher levels of power, whileproviding higher efficiency. Indeed, it is able to overcome thelimitations of the oscillating voltage due to the breakdown voltagethanks to a transformer-based power combining technique. The efficiencyis further increased thanks to the current-reuse approach. Finally, themixed stacked-interleaved configuration that is proposed for thetransformer implementation allows low-area consumption to be achieved.The transformer structure is inherently suited for (integrated) galvanicisolation, provided that a proper dielectric layer is used. Moreover, itis easy to obtain a high voltage boosting ratio between the secondaryand the primary side by taking advantage of the number of turn ratio.

It is worth noting that when inductive coupling is adopted betweenprimary coils, as in the configurations shown in FIGS. 2, 3 and 4, theequivalent resonator inductance, L_(eq) is increased according to thefollowing expression:

L _(eq)=(L _(P1,3) +L _(P2,4))·(1+k _(P))

where k_(P) is the magnetic coupling factor between the primary coils.This achievement allows obtaining a significant area reduction comparedto no-coupled coils. The proposed invention can be implemented indifferent approaches: in a monolithic solution, using a post-processingfor the dielectric and the secondary coil, using two face-to-face dicewith a post-processing for the dielectric, as the approach described inUnited State Patent Application Publication No. 2012/0256290(incorporated herein by reference) or using a system-in-package approachwith a post-processed transformer according to the Analog Device Inc.isoPower® technology.

A non-limiting design implementation of the apparatus shown in FIG. 2 isreported below for a typical 0.35-μm CMOS process. Consideringf_(osc)=250 MHz, f_(sync)=500 MHz, Ipulse=1.5 mA, VDD=3 V,L_(P1)=L_(P2)=L_(P3)=L_(P4)=5 nH, L_(S1)=L_(S2)=10 nH,k_(P1,3)=k_(P2,4)=k_(P)=0.6 (i.e., magnetic coupling factor betweenprimary coils 51), k_(P1,3 S1)=k_(P2,4-S2)=0.8 (i.e., magnetic couplingfactor between primary coils 51 and secondary coils in the output stage70), C1=C2=17 pF (excluding the parasitic capacitor of active devicesM1-M4), C3=C4=10 pF, LOAD=60 Ω, R3=R4=1 kΩ. The circuit behavior can beexplained as the superposition of two in-phase oscillators (i.e., NOSCand POSC) in which, as in classical cross-coupled topologies,transistors M1-M2 and M3-M4 provide the non-linear negative conductancerequired to sustain the steady-state oscillation. The cross-coupledconnection in the NOSC is guaranteed by the high-pass RC circuit formedby R3-C3 and R4-C4, respectively, thus allowing a bias terminal Pbias tobe available for the connection of the biasing/synchronizationcircuitry. The oscillator resonant tanks are the LC parallel networksmade up by L_(P1,) L_(P2), C1 and L_(P4), C2 for the NOSC and POSC,respectively. The tanks are tuned at about f_(osc) and therefore thedifferential voltage waveforms at the output terminals (i.e., OUT1 andOUT2) of each oscillator are forced to be almost sinusoidal at f_(osc).The presence of magnetic couplings between primary coils 51, increasesthe equivalent inductance according to the following expression:L_(eq)=(L_(P1,3)+L_(P2,4))·(1+k_(p)). The phase-relationship betweenVout NOSC (i.e., the voltage between the terminals OUT1-OUT2 of theoscillator NOSC) and Vout POSC (the voltage between the terminalsOUT1-OUT2 of the oscillator POSC) is due to the primary couplingconfiguration, as well as the common-mode synchronizing signal atf_(sync) (i.e., at 2 times f_(osc)). Therefore, the currents forced atthe primary coils are at the same time all increasing or all decreasing,and hence the fluxes generated at the primary coils. It follows that thesecondary coils will catch this flux (separately, i.e. L_(S1) will catchthe flux generated by L_(P1) and L_(P3) and so on), forcing to the loada current proportional to the fluxes. At the secondary side the outputvoltage will be greater than Vout—NOSC or Vout—POSC. The total power atthe load is the sum of the total power applied at the primary side,except for the losses in the series resistance of the windings.

To deliver high level of power with high efficiency, transistors M1-M4have to work as switches with very low on resistances. Moreover, theloss reduction in the transformer is highly related to the availabilityof low-resistance metal layers (55, 56 and 57) to obtain highquality-factor coils.

What is claimed is:
 1. An apparatus, comprising: a first oscillatorcircuit having a first output and a second output; a second oscillatorcircuit having a third output and a fourth output; and a transformercircuit comprising: a first primary winding coupled between the firstoutput and a common node; a second primary winding coupled between thesecond output and the common node; a third primary winding coupledbetween the third output and the common node; a fourth primary windingcoupled between the fourth output and the common node; and a firstsecondary winding and second secondary winding coupled in series betweenfifth and sixth output nodes, wherein the first secondary winding ismagnetically coupled to the first and third primary windings, andwherein the second secondary winding is magnetically coupled to thesecond and fourth primary windings.
 2. The apparatus of claim 1, whereinan output power at the fifth and sixth output nodes is a combination ofa first power associated with the first and second primary windings anda second power associated with the third and fourth primary windings. 3.The apparatus of claim 1, further comprising: a first capacitor coupledbetween the first output and the second output; and a second capacitorcoupled between the third output and the fourth output.
 4. The apparatusof claim 1, further comprising: a first capacitor coupled between thefirst output and the fourth output; and a second capacitor coupledbetween the second output and the third output.
 5. The apparatus ofclaim 1, further comprising a capacitor coupled between the fifth andsixth outputs.
 6. The apparatus of claim 1, wherein the secondoscillator circuit comprises a pair of transistors coupled to the thirdand fourth outputs and having control terminal coupled together toreceive a bias signal.
 7. The apparatus of claim 6, wherein the biassignal is a synchronizing signal for synchronizing frequency and phaseof oscillator operation.
 8. The apparatus of claim 7, furthercomprising: a current source configured to generate a synchronizingcurrent pulse; and a current mirror configured to mirror thesynchronizing current pulse to the control terminals of the pair oftransistors.
 9. The apparatus of claim 7, wherein the synchronizingsignal has a synchronization frequency equal to two times an oscillationfrequency of the first and second oscillator circuits.
 10. The apparatusof claim 1, further comprising a common-mode bias terminal provided inone of said first and second oscillator circuits for receiving a pulsedsynchronizing signal.
 11. The apparatus of claim 1, wherein saidtransformer circuit is formed by a plurality of windings arranged in astacked configuration, wherein the first and second secondary windingsare formed in a first metal layer and wherein the first, second, thirdand fourth primary windings are formed in a second metal layer.
 12. Theapparatus of claim 11, wherein the first and third primary windings arecoplanar and interleaved in the second metal layer and the firstsecondary winding is positioned in alignment with the first and thirdprimary windings; and wherein the second and fourth primary windings arecoplanar and interleaved in the second metal layer and the secondsecondary winding is positioned in alignment with the second and fourthprimary windings.
 13. An apparatus, comprising: a first oscillatorcircuit having a first output and a second output; a second oscillatorcircuit having a third output and a fourth output; a transformer havinga primary winding including a first portion coupled between the firstand second outputs of the first oscillator circuit and a second portioncoupled between the third and fourth outputs of the second oscillatorcircuit, and further including a secondary winding having a thirdportion coupled in series with a fourth portion, wherein the thirdportion is magnetically coupled to the first and second portions of theprimary winding and wherein the fourth portion is magnetically coupledto the first and second portions of the primary winding; wherein thefirst and second oscillator circuits and the transformer are integratedin an integrated circuit device including a plurality of metallizationlevels; and wherein said primary and secondary windings are formed insaid plurality of metallization layers.
 14. The apparatus of claim 13,wherein an output power from the secondary winding is a combination of afirst power associated with the first and second portions of the primarywinding.
 15. The apparatus of claim 13, wherein the first portion ofsaid primary winding comprises a first winding and a second windingcoupled in series at a common node; and wherein the second portion ofsaid primary winding comprises a third winding and a fourth windingcoupled in series at said common node.
 16. The apparatus of claim 15,wherein said first through fourth windings of the primary winding areformed in a first layer of said metallization layers and said secondarywinding is formed in a second layer of said metallization layers. 17.The apparatus of claim 16, wherein said first and third windings areinterleaved and wherein said second and fourth windings are interleaved.18. The apparatus of claim 17, wherein the third portion of saidsecondary winding comprises a fifth winding; wherein the fourth portionof said secondary winding comprises a sixth winding; said fifth andsixth windings coupled in series.
 19. The apparatus of claim 18, whereinsaid fifth winding is formed in said second layer over the interleavedfirst and third windings formed in said first layer; and wherein saidsixth winding is formed in said second layer over the interleaved secondand fourth windings formed in said first layer.
 20. The apparatus ofclaim 13, wherein said first oscillator circuit includes a firstcapacitor coupled between the first output and second output; andwherein said second oscillator includes a second capacitor coupledbetween the third output and fourth output.
 21. The apparatus of claim13, wherein said first oscillator includes a first capacitor coupledbetween the first output and the fourth output; and wherein said secondoscillator includes a second capacitor coupled between the second outputand the third output.
 22. The apparatus of claim 13, further comprisinga synchronization circuit configured to generate a synchronizing signalfor application to a common mode node of at least one of the first andthe second oscillator circuits for synchronization of frequency andphase.